Close Encounters in the Rain Forest

by Stephen James O'Meara

Although rain forests cover less than 7 percent of Earth's land surface, they are home to more than 50 percent of all known species of life. In a constant fight for light, water, and nutrients, many rain forest species come to rely on each other—in ways that are both intimate and exclusive.

To Die For

In the rain forests of Central America, some 50 animal species depend on the umbrella-shaped Cecropia tree—a tropical relative of the mulberry. Cecropias are “pioneer” species that quickly invade and colonize forest clearings or other disturbed areas and thrive in the rain forest's understory and lower canopy. Mammals (such as bats and monkeys) eat the trees' fruits; sloths munch on their leaves; and birds feed on insects scurrying across their broad leaves and branches. But a unique case of “evolutionary mutualism” exists between Cecropias and a seemingly unlikely ally—ants in the genus Azteca.

Cecropias have a hollow trunk and branches, which are divided into a series of chambers by partitions. The Azteca ants set up house in these chambers, and feed, in part, on nectar that the tree produces under its leaves. The tree produces this nectar for the ants, because it's a food source they're willing to die for!

You see, in return for food and shelter, the Azteca ants will defend the Cecropia's leaves against alien invaders. The ants will bite and sting anything that tries to eat Cecropia leaves and will attack and clip any vines trying to make use of their host tree. The ants further help the tree by providing a source of nutrients from frass (insect carcasses) and other insect debris that they leave behind in the stem.

The tree and ant have learned to work together for protection and survival.

Wasps…Go Figure

More than 900 species of fig trees populate the world's tropics, and many are common to rain forests. Each is pollinated by its own specific kind of fig wasp. Unlike many flowering trees, such as the magnolia or dogwood, fig tree flowers do not blossom in the open air. Instead, they blossom in the hollow center of the fruiting structure, called a synconium by botanists and a fig by everyone else. Before ripening, the fig includes separate “male” and “female” flowers. How, then, are the flowers pollinated?

The process starts when a pregnant female wasp enters a fig through a secret hole (hidden by scales) at the top of the fig. In the process, the wasp transfers pollen from the male flowers of the fig from which she emerged to the “female” flowers of the new fig she entered. The wasp also deposits her eggs in a type of “female” flower that will not set fruit. The fruits of figs are actually nutlets formed within the fruiting structure, called a fig. The tiny fruits impart the crunch to figs and Fig Newtons. The deposited pollen not only enables the fruits to grow, but also becomes a food source for the young wasps. The adult female wasp does not leave the fig; she dies there. When the larvae mature into wasps, the wingless males hatch and impregnate the females still developing in the flowers; the males of many species then also die inside the fig. The young, pregnant females, however, emerge from the flowers and exit the fig—usually before it ripens and falls to the ground—and move on to a new flowering fig, where the entire process begins again.

Meanwhile, figs are eaten by many rain forest animals, including birds, bats, monkeys, and creatures living on the forest floor. “These animals help scatter the fig seeds to other locations in the forest,” says botanist Arthur C. Gibson (University of California, Los Angeles). “Thus, while accomplishing its own reproduction, the fig tree also enables successful reproduction by the fig wasp, and provides food for many animals of the forest.”

Vocabulary

colonize:

To found a settlement.

mutualism:

A relationship between two organisms that benefits both without harming either organism.